The invention relates to new monolithic structures with parallel passages that are plugged alternately on one end face or the other of said monolithic structures that can be used in particular as particle filters for the exhaust gases of diesel engines. The invention also relates to the production of such structures.
The exhaust gases of internal combustion engines and in particular those of diesel engines contain soots or particles that pollute the atmosphere and can be very injurious to health. Various methods have been considered for attempting to solve this problem. It is proposed in particular to collect these particles in filters that consist of porous materials that are placed in the exhaust line of the engine.
Filters that consist of honeycomb-type monoliths made of refractory materials, such as cordierite or silicon carbide, were already described in the prior art.
These monoliths comprise a number of passages that are separated by porous walls, whereby said passages are alternately plugged at one end or the other to force the gas flow to diffuse through said walls.
The monoliths of this type that are produced from cordierite have the drawback of exhibiting premature aging caused by poor heat dissipation during the combustion of the soots that are retained, combined with a weak mechanical resistance.
With regard to silicon carbide monoliths already described previously, their production is difficult.
Actually, a standard technique consists in using as a starting material two silicon carbide powders of different grain sizes that are mixed, extruded, dried and brought to a very high temperature, on the order of 2100-2500xc2x0 C., in general under an inert atmosphere; under these conditions, an abnormal growth of the largest crystals is observed, and the finest crystals undergo evaporation-condensation, which finally causes undesirable modifications of the properties of the monolith that is thus produced.
More recently, a production process was proposed that is carried out starting from silicon powder and carbon or an organic material that, by thermal decomposition, will provide the carbon that is necessary for combining with silicon. The production according to such a method also requires a final calcination stage at a high temperature, generally greater than 1650xc2x0 C. and most often ranging up to 2100-2400xc2x0 C., under an inert atmosphere.
The Monoliths
The invention proposes a new monolith that can be used as a particle filter for the exhaust gases of the diesel engines, whereby this monolith has the advantage of being able to be produced by a process in which the calcination stage calls for using neither a temperature of more than 1650xc2x0 C. nor an inert atmosphere.
The monolith according to the invention can be defined in that it consists of a solid that consists of a porous refractory material whose outside volume is delimited by a cylindrical surface (in the horizontal direction) whose generatrix rests on the two end faces of any shape, for example square, rectangular, polygonal, circular or elliptical. A number of passages that are parallel to one another and to the generatrix and that empty into the end faces of said monolith pass through the solid, whereby these passages can have a square, rectangular or triangular or else polygonal section.
The monolith of the invention basically consists of a material that comprises
70 to 97% by mass, preferably 82 to 90% by mass, of xcex1 and/or xcex2 crystallographic-type silicon carbide that has at least one particle size and preferably at least two particle sizes;
and 3 to 30% by mass, preferably 18 to 15% by mass, of at least one bonding ceramic phase in the form of a micronic powder and/or particles that are obtained by atomization, comprising at least one simple oxide that is selected from among, for example, B2O3, Al2O3, SiO2, MgO, K2O, Li2O, Na2O, CaO, BaO, TiO2, ZrO2 and Fe2O3 and/or at least one mixed oxide that is selected from among, for example, the alkaline aluminosilicates (of Li, Na or K) or alkaline-earth aluminosilicates (of Mg, Ca, Sr or Ba), clays, bentonite, feldspars or other natural silico-aluminous materials.
The material that constitutes the monolithxe2x80x94i.e., the walls that separate the passagesxe2x80x94in general has a porosity of 35 to 65%, preferably 40 to 60%.
The distribution of pores is essentially monopopulated and can be centered between, for example, 5 and 60 micrometers, preferably 10 to 40 micrometers, and even more preferably 15 to 35 micrometers.
Actually, the size of the pores depends essentially on the size of the particles of silicon carbide and that of the grains of the ceramic binder that is used, as will be described latter, in particular in connection with the procedure for production of the monoliths of the invention.
The silicon carbide that has the primary portion of the material that constitutes the monoliths of the invention preferably has several particle sizes, for example from two to five particle sizes. Thus, it is possible to use designated silicon carbides, according to the FEPA 42 F 1984 standard by the symbols of F 100 to F 1200, which corresponds to particles of a mean size of 3 to 125 micrometers. In the case of a silicon carbide with two particle sizes, it is possible to consider, for example, the presence of a major proportion (for example on the order of 90% by mass) of silicon carbide in the form of particles of 15 to 125 micrometers and a minor proportion (for example on the order of 10% by mass) of silicon carbide in the form of particles that have a mean size that is less than 15 micrometers.
The bonding ceramic phase that is present in the material that constitutes monolithic structures of the invention preferably has an overall composition that is adjusted to exhibit an expansion coefficient that is close to the one of silicon carbide, nearly 50%, preferably nearly 30% and even more preferably nearly 25%.
In general, the passages that traverse the monolith are, at each end, alternately open or closed, such that for each open passage at one end, the adjacent passages are closed, whereby the gas flow that penetrates the monolith is then forced to diffuse through the porous walls that separate the passages. For example, for the square-section passages, the end faces of the monolith have the appearance of a checkerboard.
The monolith can have a number of cells (or passages) of about 50 to 400, more particularly about 100 to 200 per square inch (or about 7.75 to 62 per cm2, more particularly about 15.5 to 31 per cm2). The passages have approximately a cross section of about 0.5 to 9 mm2, more particularly 1.5 to 4 mm2, taking into account a thickness of the walls that separate the passages of about 0.3 to 1.5 mm, more particularly 0.5 to 0.8 mm.
The monoliths of the invention have exceptional properties of mechanical resistance and thermal shock resistance.
Production of the Monolith
The monolith of the invention can be produced by any suitable operating procedure that comprises in particular a stage of mixing the components resulting in a homogeneous product in the form of a thickened paste, an extrusion stage of said product through a suitable die to form the honeycomb-type monolith, a stage for drying the monolith that is obtained, then a calcination stage, whose advantage, according to the invention, is that it is carried out at a temperature that does not exceed 1650xc2x0 C., preferably less than 1550xc2x0 C., and under an atmosphere that contains oxygen. The operation that aims at plugging every other passage at each end of the monolith can be carried out at any stage of the production, either on the crude monolith, just after the extrusion, or on the dried monolith, or on the monolith that underwent a calcination stage.
A particular operating procedure is described in more detail below.
In the first stage, the silicon carbide with a suitable particle size, the bonding phase that consists of at least one simple oxide and/or at least one mixed oxide as defined above, and in general a pore-forming agent, are mixed in a dry mill, then at least one organic plasticizer, selected from, for example, the alkyl celluloses (methyl, ethyl or carboxymethyl cellulose), polyacrylic alcohol, stearic acid, bio-polymers, polyvinyl alcohol, various gums (xanthan, arabic or tragacanth) and water are added.
The pore-forming agents that can be used are, for example, starch, cellulose, carbon black, sawdust, polystyrene, ammonium nitrate, glucose, polyethylene glycols or any organic or inorganic material in the form of a calibrated powder that can decompose into volatile products during the calcination of the material.
To produce the desired porosity for the material that constitutes the monolithic structure of the invention, a group of silicon carbides that have varied particle sizes, for example, one or more silicon carbides that have relatively large particles (in the range of sizes of 15 to 125 micrometers) and one or more silicon carbides that have relatively fine particles (of sizes less than 15 micrometers), whose diameter has an influence over the diameter of the pores, is generally used by making an addition of a proportion of about 5 to 30% by mass of at least one pore-forming agent, selected based on the desired size of the pores.
It is also possible to add at least one deflocculant agent to the mixture. It is mixed until a homogeneous paste is obtained.
The homogeneous product can advantageously be cured (aged) for example at ambient temperature and under controlled hygrometry (to prevent the evaporation of the water), for example for 24 hours.
In the extrusion stage, the paste is extruded, for example, under vacuum (typically under 15 to 20 mm of mercury) in a screw (single-screw or double-screw) extruder or piston extruder to obtain crude ceramic slabs in the form of honeycomb monoliths.
These crude ceramic slabs are then dried to a temperature from, for example, ambient temperature to 110xc2x0 C., under a controlled-hygrometry atmosphere, for an adequate time to bring their content of water that is not bonded chemically (free water) to less than 1% by mass, whereby the drying lasts for example for about twenty hours.
Any method that is known to one skilled in the art that makes it possible to meet this objective under optimum technico-economic conditions is implicitly part of the production process of the invention.
The calcination of the monolithic structure is carried out under an atmosphere that contains oxygen, in general under air, by gradually raising the temperature, for example from 10 to 50xc2x0 C./hour, by keeping it at a temperature of 1100 to 1650xc2x0 C., for example 1300 to 1650xc2x0 C., preferably 1350 to 1550xc2x0 C., for at least one hour, preferably at least two hours, then it is cooled to ambient temperature.
The plugging of every other passage on each of the end faces of the monolith is carried out by application of ceramic compositions that are known to one skilled in the art. Their composition is in general as close as possible to the composition that constitutes the monolith itself, but it is adjusted to exhibit a contraction upon firing that is smaller than that of the material that constitutes the monolith and to ensure the sealing of passages that are thus plugged after the firing, regardless of the conditions of the rise or fall of temperature to which the monolith can be subjected during its use.
The monolithic structures of the invention in general consist of elementary monoliths that are assembled by ceramic bonding according to any technique that is known to one skilled in the art to constitute the particle filter that has the desired geometry to be able to be installed in the exhaust line of the diesel engine.